If the stat is mounted down stairs then I don't know why they would mount the remote sensor there! That doesn't make sense. Usually see a Accu-stat mounted inside the unit.
Some commercial buildings especially restaurants will use remote sensors so all of the thermostats can be mounted in one place. Usually in the managers office.
This is the bathroom RTU for an upscale restaurant in Houston,
They should be located in the space but the installers got lazy and a little stupid in their placement. I don't have a problem with installing them in the return plenum but well enough away from the make-up air inlet to prevent this exact scenario.